Asthma - Tachycardia to Bradycardia

akulahawkRN, ADN, RN, EMT-P · Aug 2, 2018

Hi, Just trying to find out the mechanisms involved when a patient with "severe asthma" who has tachycardia/tachypnoea progresses to "life threatening asthma" hypotension/bradycardia. I'm confused how the sympathetic (fight/flight system) is turned off in a sense and parasympathetic system is initiated.
Is it something to do with baroreceptors/muscarinic acetylcholine receptor M2?
Appreciate the help.

Tell us how you think this happens. Think through the issue. What's happening in the vasculature and lungs that could be causing this? What defines "life threatening asthma" and what does it do to the body when it occurs? Remember that body systems are often intertwined in that when something happens to one system, it will directly affect other systems that it directly interfaces with and indirectly affects others. Don't start off diving deep into the cellular "weeds" at first because you could miss the rest of the forest.

happypetal · Aug 3, 2018

Thank you for the reply.

A pt with Severe Asthma will be anxious, cyanotic , talking in words, tachycardic, tachypnoeic, moderate muscle use. The lungs and airway would be obstructed due to oedema and inflammation and vasoconstriction due to sympathetic system. When the pt progresses to Life Threatening Asthma, the pt would be unable to talk, and would be physically exhausted and may have silent chest. Now that I think about it, the pt would not be able to inhale sufficient O2 due to obstructed/inflamed airways resulting in 'breath stacking' / gas trapping in the bronchioles. Decreased O2 --> decreased preload --> decreased cardiac output. So physically exhausted pt with low CO = hypotension/bradycardia. Am I on the right path?

akulahawkRN, ADN, RN, EMT-P · Aug 3, 2018

Thank you for the reply.
A pt with Severe Asthma will be anxious, cyanotic , talking in words, tachycardic, tachypnoeic, moderate muscle use. The lungs and airway would be obstructed due to oedema and inflammation and vasoconstriction due to sympathetic system. When the pt progresses to Life Threatening Asthma, the pt would be unable to talk, and would be physically exhausted and may have silent chest. Now that I think about it, the pt would not be able to inhale sufficient O2 due to obstructed/inflamed airways resulting in 'breath stacking' / gas trapping in the bronchioles. Decreased O2 --> decreased preload --> decreased cardiac output. So physically exhausted pt with low CO = hypotension/bradycardia. Am I on the right path?

Remember that body systems are often quite intertwined. Simply obstructing the airways themselves isn't going to cause a problem with blood flow through the lungs. This isn't a simple problem. That being said, remember that blood flow is dependent upon pressure gradients and the pulmonary circuit is a (relatively) low pressure system. What happens to the intrathoracic pressure during breath stacking? What's the effect of this on the right atria (and therefore CO)? What happens to the lungs during breath stacking? What happens to the volume of the lungs? What happens to the diaphragm?

You're pretty close with the end result... and these patients can crump quickly if you don't recognize what's happening.

happypetal · Aug 4, 2018

Appreciate the tips.

Intrathoracic pressure during breath stacking - From my understanding the intrathoracic pressure would increase as the pt uses active exhalation to expel air. So when the air leaves the alveoli in an asthmatic pt, the increased intrathoracic pressure is higher than that of the bronchial smooth muscle and will squeeze the bronchioles causing 'breath stacking' resulting in 'gas trapping'. This will result in the lungs becoming distended (barrel chest) and possibly reach their total lung capacity. I heard the analogy is like taking a breath of air then trying to expel the air via a straw - you cannot expel the air in time before the next inhalation. Expiration should be a passive process but due to the bronchoconstriction, the pt requires the use of accessory muscles to expel air.

The trapped air, increased work of breathing and hyperinflation - would lead to reduced CO2 elimination (hypercapnia) resulting in V/Q mismatch or impaired gas exchange. This can result in hypoxaemia.

Therefore, impaired gas exchange (inadequate alveolar ventilation) can lead to respiratory failure which answers my original question regarding hypotension and bradycardia I think.

What happens to the diaphragm? - It would be relaxed trying to expel the trapped air, increasing intrathoracic pressure.

Right atria - This is the part I am having trouble with. A decreased CO can lead to a V/Q mismatch but I'm not sure how. *30 mins later* So..., I have just read the technical term for gas trapping is Intrinsic PEEP and is caused due to increased expiratory resistance, like an asthmatic. Intrinsic PEEP will have an effect on atrial filling and CO overall.

I think I am getting a better understanding now.

akulahawkRN, ADN, RN, EMT-P · Aug 4, 2018

I think you're now well on your way to figuring out how severe asthma progresses into life-threatening asthma and how the latter can present to you and why we must act on this very, very quickly. I also think you're now starting to see what I mean by systems being intertwined and how a problem in one system can have effects on other systems. Very rarely do problems occur in one system and not have an effect upon other systems. It's all connected and more subtly and deeply than we realize at first.

Some of this stuff can be a very deep rabbit hole to go down. Keep learning and keep questioning the why things happen the way the do and why we do things. I guarantee you that by going through this exercise with me, you've learned something more in-depth and concretely, and you'll recall it far better than if I (or anyone else) simply gave you an answer. That's why we don't give answers here. That's why we don't do your homework for you. We help you discover the knowledge.

Also, once you know what the problem is, how it affects other things, figuring out what you must do to reverse "it" becomes a much easier exercise.

happypetal · Aug 4, 2018

I really appreciate your patience and time taken out of your day to answer my questions. Thanks heaps. I guess I just did not know what questions to ask myself or where to start.

I spent quite a while reading up on your questions and I learned much more than if I was to be given the answer like you said. Thanks again.